A differential sensor may be formed by a pair of resonators, it being possible for each of the resonators to be made by depositing microstructures on the surface of a piezoelectric substrate. Such resonators belong to the family of components of surface acoustic wave type, commonly denoted “SAW” components, according to the acronym corresponding to the English terminology “Surface Acoustic Wave”, or of bulk acoustic wave type, commonly denoted according to the acronym “BAW” corresponding to the English terminology “Bulk Acoustic Wave”.
The present invention relates to differential sensors responsive to various physical parameters, for example temperature or pressure. SAW and BAW resonators exhibit the advantage of being able to be interrogated remotely, but also of not requiring a power supply. Hereinafter, reference will be made to the interrogation of an SAW or BAW sensor, to denote the gathering of the information relating to the measurement of a physical parameter.
In a typical manner, SAW sensors, for example, may be made by depositing metallic patterns on the piezoelectric substrate, the piezoelectric material possibly consisting for example of quartz (SiO2), lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). A resonator may for example consist of interdigital transducers, around which are distributed reflectors and electrodes. Radiofrequency electromagnetic signals may give rise to the generation of a surface acoustic wave, e.g. Rayleigh wave, by the transducers, the acoustic wave then propagates within the array of electrodes and reflectors, and the transducers re-emit a radiofrequency signal which may be picked up by an antenna, and analysed by appropriate means. By design, a resonator offers a natural resonant frequency, dependent notably on the physical configuration of the resonator and on the choice of materials of which it consists. The natural resonant frequency of a resonator is also dependent on the surrounding physical parameters, such as the temperature or pressure. A differential structure makes it possible for example to carry out the measurement of a specific parameter, circumventing the simultaneous influence of other physical parameters. Such a structure furthermore allows easy calibration of the sensor. In a differential structure, one of the two resonators is commonly denoted the reference resonator, the other resonator being denoted the measurement resonator.
The measurement of the physical parameter considered is performed by assessing the difference between the natural resonant frequencies of the two resonators forming the differential sensor. Hence, it is preferable that the resonators forming the differential sensor be configured so that their respective resonant frequencies are situated in distinct frequency bands, and that the difference of the two resonant frequencies define a bijective function of the physical parameter measured, in such a way that a one-to-one relation makes it possible to carry out the measurement.
The interrogation of a differential sensor is the method by which an interrogation device emits an appropriate radiofrequency signal destined for the sensor, the radiofrequency signal then being perceived by the differential sensor via a reception radiofrequency antenna, and giving rise to the propagation of a surface acoustic wave (or bulk wave, in the case of devices of BAW type), the latter being in its turn translated into a radiofrequency signal re-emitted via an emission antenna, the latter possibly being formed by the aforementioned reception antenna, it then being possible for the radiofrequency signal thus re-emitted to be gathered and analysed by the interrogation device. The interrogation signal emitted can take the form of a brief radiofrequency pulse.
In a typical manner, the aforementioned radiofrequency signals are situated in a frequency band denoted ISM, according to the initials denoting the English terminology “Industrial, Scientific, Medical”. A first ISM band is situated around the frequency of 434 MHz, with a bandwidth of 1.7 MHz. This frequency band is particularly advantageous, notably because of the signal penetration depth that it affords, in dielectric media within which the sensors may be used. However, the relatively low passband is notably prejudicial to the signal-to-noise ratio, and therefore to the maximum interrogation distance. This phenomenon is all the more significant for differential sensors, for which the frequency bands, wherein lie the ranges of values of resonant frequency of the resonators of which they consist, must be distinct, as is explained above. A background issue related to the interrogation of SAW sensors thus pertains to the signal-to-noise ratio afforded by the method of interrogation.
Known solutions of the prior art allow the interrogation of SAW resonators.
According to a first known technique, described in the publication “Friedt, J.-M.; Droit, C.; Martin, G.; Ballandras, S. “A wireless interrogation system exploiting narrowband acoustic resonator for remote physical quantity measurement”, Rev. Sci. Instrum. Vol. 81, 014701 (2010)”, the interrogation may be carried out by a frequency scan of the interrogation signal of smaller spectral bandwidth than the width of the resonance, around the natural resonant frequency of the first resonator on the one hand, and of the second resonator on the other hand, followed by an analysis of the power of the signal received as echo, the emission frequencies for which the power of the signal received is maximal then determining the two natural resonant frequencies sought. A radiofrequency switch can make it possible to alternate emission phases and listening phases. This first technique presents notably the drawback of requiring wide-band power detectors, these devices being relatively complex and expensive.
According to a second known technique, described in the publication “Hamsch, M.; Hoffmann, R.; Buff, W.; Binhack, M.; Klett, S.; “An interrogation unit for passive wireless SAW sensors based on Fourier transform Ultrasonics, Ferroelectrics and Frequency Control”, IEEE Transactions on, November 2004”, the interrogation may be carried out by a frequency scan of an interrogation signal of larger spectral breadth than the sensor resonance width, in a frequency range covering the two natural resonant frequencies of the resonators, followed by an analog-to-digital conversion of the signal received as echo, making it possible to carry out a fast Fourier transform and an extraction of the spectrum of this signal, from which the two natural resonant frequencies sought may be deduced. This second technique presents notably the drawback of requiring significant calculation means.